Hemolymph composition of fourth instar larvae of Culex pipiens was examined to determine the effects of parasitism by Romanomermis culicivorax. Mosquitoes were reared under two different pH regimens: 4.5 and 7.3. Total alpha-amino nitrogen was significantly lower in infected mosquitoes reared at pH 7.3. However, total amino acid concentrations for hemolymph from control and infected larvae, reared at pH 7.3 were the same. Methionine sulfoxide decreased 63 percent and proline increased 2.5 times in infected mosquitoes. No significant changes in the concentrations of other amino acids were found. The role of R. culicivorax in the degradation of hemolymph proteins in infected fourth instar C. pipiens was evaluated. Electrophoresis of proteins from hemolymph of C. pipiens, infected and control, revealed no new proteins in the hemolymph of infected mosquito larvae. Protein patterns from homogenates of R. culicivorax were compared with those from infected hemolymph and no proteins of parasite origin could be detected in the host hemolymph. Appreciable proteolytic activity was found in the hemolymph of control mosquitoes, whereas, activity in the hemolymph of infected mosquitoes and homogenates of R. culicivorax was nearly undetectable.